Heat-labile toxin (HLT) is a potent toxin produced by Bordetella species. This toxin induces tissue damage and may contribute to damage observed after infection. Only one other toxin, tracheal cytotoxin, is known to be produced by all four species in the genus Bordetella. Since the different species produce similar diseases albeit in different hosts, HLT may play an important role in the pathogenesis of the organisms. We have initiated a program to examine the structure and function of this toxin. A cell culture assay for HLT was developed. In this assay, the ability of HLT to induce contraction of vascular smooth muscle cells is measured. The results obtained by this method, which allows for detection of a little as 0.6 ng/ml of the toxin, correlated well with those obtained with in vivo assays. The ability of HLT to induce skin lesions in guinea pigs was found to be inhibited by lipids. Long-chain fatty acids were found to inhibit HLT; however, fatty alcohols, neutral lipids, phospholipids, cholesterol, and prostaglandin had no measurable effect on HLT action. These data suggest that the ability of HLT to induce skin lesions in animals may depend, in part, on the free fatty acid content of the skin layer.